Third Day Poster Session, 17 June 2010 - NanoTR-VI

P
P mP
P vs.
P =
P,P
P (1)
P and
Poster Session, Thursday, June 17
Theme F686 - N1123
The Effect of Film Thickness on the Optical Properties of ZnO Thin Films
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UBanu ErdoanUP P*, Derya BaharP P, Göknil BabürP P, Sinan DikenP P, Aye Tuba TermeliP P, Sava SönmezoluP
Pand Güven ÇankayaP
1
PDepartment of Physics, Faculty of Arts and Science, Gaziosmanpaa University, Tokat 60250, Turkey
Abstract-ZnO thin films were deposited on soda lime glass substrates by sol–gel spin-coating technique. The opticalproperties of ZnO thin
films are investigated for different film thickness. It showed that the optical band gaps of thin filmdecreased with increasing film thickness, on
the contrary, transmission of thin films increased with increasing film thickness.
1
Zinc oxide (ZnO) is one of the most important group II–VI
semiconductor materials. ZnO is one of the most promising
materials for the fabrication of the next generation of
optoelectronic devices in the UV region and optical or display
devices. As a matter of fact, simultaneous occurrence of both
high optical transmittance in the visble range, and low
resistivity make ZnO an important material in the manufacture
of heat mirrors used in gas stoves, conducting coatings in
aircrafts glasses to avoid surface icing, and thin film
electrodes in amorphous silicon solar cell [1]. ZnO is also used
to fabricate piezoelectric micro-force sensor chip[2].
In this paper, we report the investigation of ZnO thin films
prepared by sol-gel spin coating process using zinc acetate
(ZnAc). The optical characterization is investigated for
different film thickness using Perkin Elmer Lambda 35 UV-
VIS Spectrometer at room temperature.
Transmittance (%)
100
80
60
40
20
5 layer
10 layer
15 layer
20 layer
0
200 400 600 800 1000 1200
Wavelenght (nm)
Figure1. UV–VIS spectra of the ZnO thin film for different film
thickness
In order to prepare a ZnO solution, first, 3.35gr zinc acetate
(Zn(CHR3RCOO)R2R·2HR2RO, Merck), used as a precursor, was
dissolved in 50 ml ethanol [CR2RHR6RO, Merck] and stirred for 5
0
min at 60 P PC in a magnetic mixture. Then, 5 ml glacial acetic
acide [CR2RHR4ROR2R, Merck] and 1.5 ml hydrochloride acid (HCl,
Merck) were added in the solution, and the final solution was
subjected to the magnetic mixture for 2 h. Here, glacial acetic
acid and hydrochloride acid were used as an inhibitor to slow
down the zinc acetate fast hydrolysis. Prior to the coating
process, the glass was washed with water, ultrasonically
cleaned in ethanol for 20 min, and in acetone for 20 min,
respectively. The deposition was carried out at a spinning
speed of 3000 rpm for 30 s. The spin coating procedure was
repeated for 5 layers, 10 layers, 15 layers and 20 layers on glass
substrate at 300 ºC temperature.
Figure 1 shows the UV–VIS spectra ZnO thin films for
different film thickness in wavelength range 300–1100nm.
The transmission of thin films increases with increased the
film thickness. The diffrerence related to the grain boundaries
are observed as the film grows thicker. [3].
(hv) 2 (eV/m) 2
5
4
3
2
1
5 layer, E g
= 3.66 eV
10 layer, E g
= 3.59 eV
15 layer, E g
= 3.63 eV
20 layer, E g = 3.64 eV
0
2 2.4 2.8 3.2 3.6 4
Photon energy (eV)
2
Figure2. The plot of (hv)P P vs. hv of the ZnO thin film for different
film thickness.
The optical band gap of the film was calculated by the
following relation [4]:
r
(hv) = A (hv - ERgR) P
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where A is an energy-independent constant between 10P
8 -1
10P
P, Eg is the optical band gap and r is a constant, which
determines type of optical transition, r = 1/2, 2, 3/2 or 3 for
allowed direct, allowed indirect, forbidden direct and
forbidden indirect electronic transitions, respectively[4]. The
1/r
(hv)P hv curves were plotted for different r values and
the best fit was obtained for r = ½. The film at various
annealing temperatures shows a direct allowed transition. The
optical band gap was determined by extrapolating the linear
2
portion of the plots to (hv)P 0. The optical band gaps of the
thin film were found to be 3.66, 3.59, 3.63 and 3.64 eV at 300 ºC
annealing temperature. The thicknesses of ZnO film were also
determined from transmittance measurements in Figure1 and
found to be 302, 308, 385 and 695 nm, respectively. The optical
band gap decreases with the increasing film thickness.
In summary, the analysis of the transmission spectra
shows that ZnO thin films are transparent in the UV-visible
region irrespective of the film thickness. This work was
partially supported by the Scientific Research Commission of
Gaziosmanpaa University (Project No: 2009/29).
*Corresponding author: bnrdgn@gmail.com
[1] M. Suchea, S. Chiritoulakis, K. Moschovis, N. Katsarakis, G.
Kiriakidis, Thin Solid Films 515 (2006) 551.
[2] S.S. Lee, R.M. White, Sens. Actuators A 71 (1998) 153–157.
[3]. C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and
Chemistry of Sol Gel Processing, Academic Press, New York, 1975,
p. 87.
[4] . Tauc, Mater. Res. Bull. 5 (1970) 721
6th Nanoscience and Nanotechnology Conference, zmir, 2010 621